The m_pages hash map in FontFallbackList is defined as follows:
typedef HashMap<int, GlyphPageTreeNode*> GlyphPages;
GlyphPages m_pages;
This uses the default HashMap minimum table size of 64 buckets which is quite wasteful in many cases.
One idea is to lower the minimum table size using hash traits. A patch will follow that lowers it to 16, which reduces memory consumption by ~900 kB when loading the full HTML5 spec.